Mustrum_Ridcully
Legend
That's a sandbox campaign? If it is - why not have the characters go on - apparently, their adventure isn't really over yet, and they have to face even stronger foes despite the fact that none of the are fully rested and healed.IME, players have an easy time with "PCs should be heroes" and a harder time with "I know the rules say you can cast Cure Light on that guy 'cause it'll interfere with his 'final words'." or "I know the rules say you can be bopping up tomorrow, and I know that you've heard the orc army is marching on the village where you live, but you really need to take a week's rest before doing anything about it."
How would your rules benefit a group in that situation, either? Why are they not pressed by the situation - they can, after all, still go on, even if they only have a subset of their regular hit points. They are still partially healed, and can survive a few hits.
If you really need a rules construct, you could introduce a "Quest Aftermath" - after you have made your quest, you are fatigued until you have rested for 1 week. And create a magical ritual/spell that removes these fatigue.